Diazotypy is a copying process for making copies from translucent originals by exposing then to ultraviolet light while in optical contact with a diazotype copying sheet, and developing with ammonia, or heat, or a liquid developer to form a print dye.
The diazotype process is based on reactions typical to the components used and on the time controlled sequence of these reactions. The main function components of the diazotype process are:
1. Aromatic diazo compounds of more or less pronounced yellow color which absorb ultraviolet light to undergo a photolytical decomposition to colorless products. The diazo compounds can be mixed with azo couplers when maintained in acid environment to prevent precoupling and then by changing the pH from an acid to an alkaline environment, the coupling reaction can take place to produce an azo dye.
2. Couplers are aromatic compounds with phenolic OH groups with or without other substitutions, or other organic compounds containing activated methylene groups. The couplers are colorless compounds which can be mixed with the diazo compounds as noted above without reacting with them while in an acid environment but by changing the acid environment to an alkaline environment the coupling reaction takes place to form an azo dye. Also, when employing a liquid development system, a coupler can be added to the developer solution rather than mixed with the diazo compound as a dry component.
Other components may be added to the diazotype layers to obtain various effects by controlling the reaction rates of the coupling reactions.
The light sensitive diazo compounds, the dye forming couplers, and the azo dyes forming the print image are all low molecular weight compounds which do not polymerize and do not resinate. The main performance parameters of a diazotype material are the light sensitivity and print dye strength. Both depend substantially on the distribution of the diazo compounds and couplers both laterally and in depth in the surface zone of the base support sheet and on the compatibility and dyeability of the carrier material. It can be expected that the eveness of distribution will be greatly enhanced by an appropriate matrix material for the diazotype compounds similar to silver halide photography where gelatin is used as the matrix. The sensitometric characteristics of the light sensitive layer are greatly influenced by the carrier, for example, see Light Sensitive Systems by Kosar, page 32, published by John Wiley & Sons Inc., 1965.
The best known matrix in the silver halide photographic process is gelatin. It is also known that the quality of the gelatin and the nature of the dispersion of the silver halide grains in the gelatin control the performance of the photographic material. The gelatin is the matrix in which the light sensitive photographic material is embedded.
The diazotypy uses mostly paper as a base support material for economical and practical reasons. The paper fibers of diazotype base papers have undergone a sizing process which makes them sufficiently receptive to the diazotype components when applied from an aqueous solution without excessive absorption into the fibrous structure of the base support sheet. The fibrous structure of the base paper, however, makes it impossible to obtain a uniform grainless print because the degree of uniformity depends strongly upon the degree of sizing which has been applied to the base paper. Also, the base paper can have a strong deteriorating effect on the diazotype product due to the potential contaminating impurities contained in the paper itself. The in-depth distribution of the diazotype components forcibly follows the fibrous structure by capillarity which makes uniformity impossible.
Numerous efforts have been made to remedy these problems and disadvantages by introducing potential matrix materials for diazotypes, for example, gelatin and other hydrophilic colloids such as starch, cellulose-ethers, alginates, polyacrylic acid, casein, polyvinylpyrrolidone and ethylene-oxide polymers. However, these materials do not produce the desired results for reasons for example, of limited compatibility, of excessive viscosity, insufficient flowability. The use of polyvinyl acetate dispersions resulted in a number of problems and disadvantages such as limited compatibility, and softness of print surface among others. Moreover, most of these coating applications cannot be evenly dispersed nor readily or uniformly applied to the paper surface. The most successful achievement has been the introduction of precoating (TAPPI Vol. 48, No. 3, August 1965). Particularly the use of selected non-colloidal size pigment precoats reduced the influence of the paper base on the performance of the diazotype and results in more uniform print surfaces and better print contrast which is desirable for line reproduction work to which diazotypes were mostly restricted. These precoating applications, however, cannot be controlled to obtain low contrast diazotypes which would be useful for continuous tone reproductions. Such precoat layers cannot be increased beyond an optimum thickness since the diazo components tend to distribute unevenly into the deeper recesses of the precoat layer. In other words, the precoat layers are more like planar layers and do not provide a steric matrix for the diazotype components.
Heretofore, a preferred matrix for diazotype components has been cellulose acetate or cellulose aceto butyrate; they form either self-supporting films or can be coated to another base support. The diazotype components are compatible with these materials. However, diazotype products with these components as a matrix are expensive to produce because of the high cost of the materials and the special preparation and coating techniques that are necessary which require the use of organic solvents as dispersing agents to apply these water insoluble materials to the base sheet.
The diazotype process is a positive, working, bleach-out process. It works by decomposing the diazo material which is exposed to ultraviolet light thereby forming a print in those areas where the opaque image of the master was placed. Thus, flash exposure of diazotype layers results in decomposition of the diazo components and more or less proportional reduction of potential print dye formation. On the other hand, the silver halide process uses light exposure of the sensitive layer to generate a latent image and it is the concentration of the light which contributes to the strength of the image dye formation.
The sensitometric characteristics of diazotype layers are quite different from those of silver halide layers and accordingly it was generally assumed that diazo-type layers are useful only for line work reproductions (Kosar - Light Sensitive Systems, page 302). Their sensitometric curve has no or only a very short straight line portion with a short toe section that turns quickly into a convex zone and flattens out rapidly at higher densities. Accordingly, the greatest reprint contrast ratio of diazotypes occurs in the upper part of the toe section. Kosar in his article also mentions that unlike silver halide photographic layers, the sensitometric curve of diazotype layers can only be affected very slightly by the formulation of the sensitizing solution, the exposure or development.
The principal application for diazotype material is for line work reproduction and the simplicity of the process and its economy when compared with other reproduction processes of more recent dates resulted in its wide use. Improvements of the diazotype process were directed in many cases towards improving its characteristics for line reproductions by widening the high contrast zone of the sensitometric curve. As indicated above, however, no more than a slight improvement can be obtained by reformulation of the sensitizing solution, and only the introduction of the precoating process resulted in improvements with a substantial widening of the high contrast zone of the sensitometric curve. Excellent diazotype copies are thus obtained from line work masters on clear or matte film. Also, satisfactory reproductions are obtained from such masters on transparent paper of homogeneous translucency.
Very poorly readable copies are obtained, however, from typewritten letters on common commercial stationary paper because of its uneven formation and because of intentional decorative effects of the stationary such as cockle in onion skin, water marks, etc. The natural or artificial patterns of inhomogeneity optically constitute differences in transmission density of the sheet and thus have an additive effect on the image dye pattern where they overlap with it, and an imaging effect of their own pattern in the otherwise image-empty areas of the sheet. Because the transmission densities of these patterns are mostly rather close to the relatively low transmission densities of the typewritten information the patterns only reproduce on the diazotype in the high contrast range of the toe zone of the sensitometric curve. The result for the viewer of the copy is a weak image line among a rough pattern of specks of similar reflection density. This shortcoming has discouraged the use of diazotypes for copying from letterhead masters or masters on paper with wild formation patterns in favor of the use of more expensive reproduction methods.
Lowering the contrast ratio of diazotypes towards a ratio of one particularly in the upper toe zone and extending it towards the zone of higher densities would overcome these problems and disadvantages and permit the use of diazo reproduction materials in applications which heretofore have not been feasible with diazotypes.
Various approaches have been made in the past with such an aid in mind; however, this work has been restricted to diazotype film coating layers; for example, Dutch Patent No. 80,603 suggests the use of a mixture of two diazo compounds with different absorption characteristics for printing light in order to obtain a lower print contrast for tonal reproductions. Also, U.S. Pat. No. 3,484,241, suggests the use of two diazotype layers with the diazo components having different sensitivities. Also, U.S. Pat. Nos. 3,069,268; 3,365,269; 3,498,791 and 3,661,591 recommend the use of particular ultraviolet absorbing substances which are applied in single or double layers to the base sheet and in which these ultraviolet light absorbing substances are spread evenly through the depth of the layers of consistent thickness.
These teachings have been found useful in diazo micro films but applications to paper have not been successful, probably because the thickness of the diazo coating layers on paper is never consistent and the coating layer tends to intermingle with the fibrous structure of the paper.
Another U.S. Pat. No. 2,603,564 employs certain pigments or fillers in high concentrations and applies them to the diazo layers intermingled with hydrophobic organic resin binders in an attempt to obtain a lower contrast ratio for diazo reproductions. However, these organic resin binders are soluble in organic solvents only and, therefore, require an organic solvent system. Such a system has the disadvantage that it is difficult to homogeneously distribute the pigments in the diazo coating solution and it is difficult to avoid sedimentation. Also, to apply such a system a more expensive base paper must be employed with solvent holdout and such a system would require the use of high viscosity solutions and modification of the coating procedure.
An upgraded diazo reproduction material has been obtained through the use of baryta coated base paper. Diazo sensitized baryta coated base paper produces excellent diazo-type prints but the sensitometric curve of such layers still show the high contrast ratio in the toe zone and good half tone prints are only obtained through the use of screened master film originals. Heavy weight base stock is necessary when using baryta coated paper since the coating layers have a weight of at least 30 grams per square meter. These heavy baryta coating layers are thick and can easily fracture when folded and, further, such thick layers cause curl problems which are difficult if not impossible to overcome. Also, the high cost for making baryta coated materials limits their use to only special applications.
The need for diazo reproduction materials which are inert against handling and are waterfast has always been apparent. The diazo materials which can be used for these purposes are diazotype films and lacquer coated diazotype papers which may be translucent or opaque. However, these particular diazotype materials, although the reproductions obtained are of high quality, are expensive to make and their manufacture is time consuming. To perform the lacquering process on paper with a regular calendar finish is difficult to achieve since pinholes often occur and these pinholes form uneven tonal qualities in the diazo print. These pinhole or crater effects can be minimized when a baryta coated base is used for the lacquering process. It is necessary to employ thick lacquer layers to avoid penetration of components from the organic solvent solution of the diazo components into the base which would result in unevenness in the tonal qualities of the diazo reproduction. Incorporation of diazotype components into organic solvent lacquer systems result in a yellow-brown discoloration of the print background. Such type of discoloration does not occur, or only to a much lesser degree from the use of aqueous sensitizing solutions, which, however, do not take to lacquer layers. Since the above diazo reproduction materials are expensive they are not widely used throughout the industry.
The diazotype process is used to make copies of different colors such as blue, yellow, brown, red, black, etc., but is not used to make multi-color copies. A two color process for diazo reproductions with shade differences between the full tones and intermediate tones has been suggested in U.S. Pat. No. 2,542,715 and No. 2,542,716. To accomplish this a combination of slow and fast acting azo couplers are used, for example, diazo reproduction materials which have been sensitized with 2-diazo-1-napthol-5-sulfonic acid and phloroglucinol as the coupler, produce prints with blue shades in full tones and light red shades in the intermediate tone areas.
In the past multi-color diazo reproductions could only be achieved through a composition of transparent overlay films. Attempts have been made to achieve multicolored diazo reproductions on a single base sheet but have been unsuccessful. Attempts have been made in which the diazo print in one color is developed and then the same diazo print is resensitized with a different diazo sensitizing composition for a different color and printed and developed and then a third resensitizing and so forth have been applied to achieve a multi-color print. These attempts have failed because each time the diazo reproduction is resensitized the underlying dye is not sufficiently resistent to the acid sensitizing solution and may decompose or bleed out or both. To overcome this problem the resensitizing solution was carried out employing organic solvent solutions. However, it is necessary to anchor the developed top layer to the under layer whereby these solvents tend to dissolve out the azo dye of the underlying print image.
Image transfer is very useful for reflex copying processes and for photomontage. Conventional diazotype reproduction materials do not lend themselves to such applications.
Diazotype layers on paper are intermingled with the fibrous paper surface and are resistant to contact transfer to a receiver sheet unless a solvent for the diazo, such as water, or solvents for the azo print dye are used for the transfer which, however, results in strong dye bleeding and image blurring.
Diazotype layers which are imbedded in a wax type coating are more easily contact transferable through pressure or solvents or swelling agents for the wax layers but such composites are impractical for diazotype reproduction media because the coating layer is insufficiently resistant to manual handling and to processing of the sheets in printing machines, and moreover the ammonia development is strongly affected because the waxy layer is not pervious enough to ammonia and water vapors.